In a modern passenger aircraft the passenger cabin or sub-regions of the passenger cabin, the hold or sub-regions of the hold, the cockpit and the crew rest rooms form various climate zones that may be air-conditioned by means of an aircraft air conditioning system while the aircraft is operating both in the air and on the ground.
When the aircraft is operating in the air, the aircraft air conditioning system moreover ensures that the passenger cabin and at least sub-regions of the hold are maintained at a pressure that is higher than the reduced ambient pressure at cruising altitude of the aircraft. When the aircraft is flying, hot pressurized bleed air removed from the engine compressors or auxiliary engine compressors and/or ambient air is usually supplied to the aircraft air conditioning system. When the aircraft is operating on the ground with the engines switched off, on the other hand, the air conditioning system is supplied as a rule with ambient air. When the aircraft is operating on the ground, it is moreover possible to air-condition the various climate zones of the aircraft by means of an external aircraft air conditioning system that is connectable to the aircraft. The external aircraft air conditioning system is likewise usually supplied with ambient air.
In the air conditioning units of the aircraft air conditioning system and/or of the external aircraft air conditioning system the bleed air and/or the ambient air is cooled down to a desired low temperature. The air conditioned in the air conditioning units is conveyed as cooled fresh air into a conventionally internal mixer of the aircraft, where it is mixed with recirculation air that is extracted from the passenger cabin. The mixed air produced in the mixer from cold fresh air provided by the air conditioning packs and recirculation air extracted from the aircraft cabin is finally used to condition the various climate zones of the aircraft.
In order to prevent an accumulation of moisture in the interior of the aircraft and an icing-up of the air-carrying lines of the aircraft air conditioning system while the aircraft is flying, the fresh air supplied to the mixer of the aircraft air conditioning system should not exceed a specific moisture content. Since, as the air temperature drops, the humidity-absorbing capacity of the air also drops, a dehumidification of this air is achieved by the cooling of the bleed air and/or the ambient air in the air conditioning units of the aircraft air conditioning system or of the external aircraft air conditioning system. However, particularly when the aircraft is operating on the ground in warm and humid ambient conditions, an extreme cooling of the ambient air supplied to the aircraft air conditioning system or to the external aircraft air conditioning system is often needed to guarantee adequate air dehumidification. As a result of this, the air conditioning packs of the aircraft air conditioning system or of the external aircraft air conditioning system have to produce a very high cooling capacity, thereby leading to a high primary energy consumption of the aircraft air conditioning system or of the external aircraft air conditioning system.